There is a conventional method of employing an diffractive optical element for reducing chromatic aberrations. In this method, a diffractive optical element is provided on a lens surface, or it is provided not on a lens surface but at another part of an optical system.
A diffractive optical element is an optical element which has a grating with a large number of very fine and evenly spaced parallel grooves or slits, the number being several hundreds per minute interval (for example, per 1 mm). This optical element has a characteristic that a beam of light, before passing through the optical element, goes through a change in direction which is defined by the wavelength of the light and the pitch (clearance) of the grooves or slits of the diffraction grating. Such diffractive optical elements are used in various types of optical systems. For example, a diffractive optical element is used as a lens to focus light rays with a specific order of diffraction for reducing chromatic aberrations.
In recent years, a so-called multilayer diffractive optical element has been proposed as such a diffractive optical element. This type of diffractive optical element comprises a plurality of diffractive elements, which are placed one over another, each element having a surface formed with a sawtooth-like cross section. This type of diffractive optical element displays a high diffraction efficiency for almost all region in a desired wavelength range (for example, the visible light range), so it offers good wavelength characteristics.
A well known multilayer diffractive optical element is a bonded bilayer diffractive optical element, in which, for example, two types of diffractive optical elements made from different materials are bonded together at their identical diffraction gratings (for example, refer to Japanese Laid-Open Patent Publication No. H9(1997)-127321).
For fabricating such a bonded bilayer diffractive optical element, there is a replica method. In this method, a photo-setting resin is dropped on a base plate, and the dropped resin is stamped with a metal die that has a relief whose shape is counter to a desired grating. Then, the stamped resin is cured by a light beam from a light source for setting the resin in the shape of an optical element before it is removed from the plate and the metal die.
In a case where a bonded bilayer diffractive optical element is fabricated by such a replica method, a first ultraviolet-curable resin and a second ultraviolet-curable resin, which are different from each other, are prepared as materials for forming the optical element. Firstly, the first ultraviolet-curable resin dropped on a base plate is stamped with a metal die having a counter relief of a desired diffraction grating, and this resin is treated with an ultraviolet light beam. After the resin has cured, the metal die is removed from the resin, which is now set as a first diffractive element. Secondly, the second ultraviolet-curable resin is dropped on the surface which has the diffraction grating of the first diffractive element, and this resin is stamped with a metal die for surface formation and treated with an ultraviolet light beam for curing. Finally, the surface-forming metal die is removed from the latter resin, which is now set as a second diffractive element. As a result of this procedure, an diffractive optical element is created, comprising these two elements, one from the first ultraviolet-curable resin and the other from the second ultraviolet-curable resin, bonded at the common diffraction grating.
In this fabrication method, which is a replication technology, the ultraviolet-light irradiation for curing the second ultraviolet-curable resin is performed from the side of the first ultraviolet-curable resin. In general, since a photo-setting resin such as an ultraviolet-curable resin has a relatively low transmittance, thick part of an element made from a photo-setting resin has a transmittance lower than the other part. This tendency is remarkable especially in short-wavelength region.
Because of this phenomenon, when the second ultraviolet-curable resin is irradiated through the first ultraviolet-curable resin, there is a gradient in the intensity of light through the layer of the second ultraviolet-curable resin. This light-intensity gradient causes differences in cure shrinkage, which results in ripples or a so-called embossing on the surface of the second ultraviolet-curable resin. This embossing of ripples has a periodicity as it is caused by the up-and-downs of the diffraction grating. As a result, this embossing itself exhibits somewhat an effect of a diffraction grating. This means that the diffractive optical element fabricated by the replication technology acquires a diffraction grating on the surface of the second ultraviolet-curable resin in addition to the originally intended diffraction grating, which is created at the boundary surface of the first and second ultraviolet-curable resin layers. This is a problem in achieving a desired optical performance.